Rhodium tris complexes

In 1998, Enders et al. reported the use of the rhodium(cod) complexes 54a-f containing chiral triazolinylidenes in the same reaction [41]. Complexes 54 were prepared in THF in 65-95% yield, by reaction of the tri-azolium salts with 0.45 equiv of [Rh(cod)Cl]2 in the presence of NEts (Scheme 31). The carbene ligand in such complexes is nonchelating with possible hindered rotation around the carbene carbon-rhodium bond. Due to... 

Systems which fulfil these conditions are tris(2,2 -bipyridyl)rhodium complexes [63] and, more effectively, substituted or unsubstituted (2,2 -bipyridyl) (pentamethylcyclopentadienyl)-rhodium complexes [64], Electrochemical reduction of these complexes at potentials between — 680 mV and — 840 mV vs SCE leads to the formation of rhodium hydride complexes. Strong catalytic effects observed in cyclic voltammetry and preparative electrolyses are... 

The absorption spectra of tris-polypyridyl Rhodium(III) complexes are characterised by several intense Ligand Centered (LC) absorption bands in the UV. Neither MC absorption bands, nor CT bands are observed in the visible region of the spectrum in contrast to their Ruthenium analogues. This makes tris(polypyridyl)Rh(III) complexes formed with bpy and phen practically colorless [1]. 

The behaviour in room-temperature fluid solutions of excited Rhodium(III)-polypyridyl complexes remains unclear. These compounds are weak emitters, and perhaps because of this, contradictory reports on the room temperature emissions of Rh(bpy)3 and Rh(phen)3" have been published. Indelli et al. [129] detected the emission at 588 nm (dd ) and 455 nm nn ) for Rh(phen)3 while Nishizawa et al. [127] observed only the nn emission at 455 nm. The tris-polypyridyl Rhodium(III) complexes photodissociate, giving rise to the loss of a ligand [130], as is expected when the MC state can be populated. 

Rhodium(I) complexes have also been shown to promote metallo-ene type reactions efficiently (Scheme 7.14) [26]. Typically, the reaction of 2,7-octadienyl-l-carbonate 27 is carried out using the RhH(PPh3)4-tris(2,4,6-trimethoxyphenyl)phosphine system as the catalyst in acetic acid at 80 °C for 1-1.5 h, to give the corresponding l-exo-methylene-2-ethenylcyclopentane 28 in high yield. 

P-31 NMR was a powerful tool in studies correlating the structure of tertiary-phosphine-rhodium chloride complexes with their behavior as olefin hydrogenation catalysts. Triphenylphosphine-rhodium complex hydrogenation catalyst species (1) were studied by Tolman et al. at du Pont and Company (2). They found that tris(triphenylphosphine)rhodium(I) chloride (A) dissociates to tri-phenylphosphine and a highly reactive intermediate (B). The latter is dimerized to tetrakis(triphenylphosphine)dirhodium(I) dichloride (C). 

In 1965 Wilkinson invented the rhodium-tris(triphenylphosphine) catalyst as a hydrogenation catalyst [60]. It still forms the basis for many of the chiral hydrogenations performed today. The most effective homogeneous hydrogenation catalysts are complexes consisting of a central metal ion, one or more (chiral) ligands and anions which are able to activate molecular hydrogen and to add the two H atoms to an acceptor substrate. Experience has shown that low-valent Ru,... 

The complexes can also be prepared from other monomeric rhodium(I) complexes. Both the triphenylphosphine and the tri(/>-tolyl)phosphine complexes can be prepared by refluxing solutions of the appropriate [RhCl(PAr3)3] complex in an inert atmosphere (equation 23).68 70 Alternatively one ligand of an [RhXL3] complex can be oxidized (equation 24).87,88... 

The dihydrido complexes (Table 62) can be obtained by the oxidative addition of molecular hydrogen to rhodium(I) complexes (equation 186).10,119,922""926 The tri(f-butyl)phosphine complexes can be prepared either from the chlororhodium(I) complex,923 or rhodium trichloride.927 The former method seems more reliable since the latter reports the complex as a matt green substance, a color uncharacteristic of tertiary phosphine rhodium(III) complexes. Indeed, Masters and Shaw report that the related tertiary phosphines PBu2R (R = Et, Pr) give green rhodium(II) complexes in this reaction (see Section 48.5.2.1 above).268,269... 

An unstable tri(perfluorophenyl)phosphine complex can be obtained by oxidative addition of chlorine to the dimeric rhodium(I) complex. The ethyldiphenylphosphine complex prepared in this way is more stable (equation 202). 

One interesting reaction undergone by the tri(styryl)arsine complexes is the bromination of the C=C bond by bromine in CC14 (equation 217).1013 Similar behavior970 is exhibited by the few tertiary stibine complexes (Table 75) that have been isolated. Few physical properties of these complexes have been investigated, but the 121 Sb Mossbauer parameters for both rhodium(III) complexes and the free ligands have been determined.1016... 

Bidentate oxygen ligands form numerous rhodium(III) complexes. Several tris(jS-diketonato) complexes have been prepared from rhodium(III) nitrate (equation 35). The products are extraordinarily stable. They can be resolved into their optical isomers, and survive nitration and formylation reactions (equations 36 and 37). The tris(oxalato)rhodate(III) ion has also been resolved, but the enantiomers undergo slow racemization. Reaction of this complex with refluxing chloric(VII) acid leads to m-[Rh(ox)2(H20)2], which can be converted into cis- or frani -[RhX2(ox)2] complexes. 

The tridentate ligands (38)-(40) form rhodium(I) complexes. The complexes of the first two ligands readily undergo oxidative addition to form rhodium(III) complexes. The complex [RhCl(38)] also adds either SO2 or BF3 to form pentacoordinate rhodium(I) complexes. The tetraden-tate ligands (41) (Z = P, As) and (42) and the hexadentate ligand (43) form both rhodium(I) and (III) complexes. By contrast, the tri(tertiary arsine) ligand (44) fails to reduce hydrated rhodium trichloride and forms both fac- and mer-trihalorhodium(in) complexes. 

Raymond and co-workers have synthesized and separated optical and geometrical isomers of simple tris hydroxamate chroniium(III) and tris phenolate chromium(III) or rhodium(III) complexes and assigned the absolute configurations of these isomers based on criteria such as chromatographic behavior due to differences in dipole moment, theoretical symmetry considerations, and X-ray crystallographic data 174). The absolute configuration of isomers of chromium(III) complexes of ferrichrome, ferrichrysin 175), ferrioxamine B and Dx 176), rhodotorulic acid 177), enterobactin 178),... 

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